The objective of this invention is to suggest efficient distillation schemes to separate feed mixtures containing three or more components into product streams each enriched in one of the components. Generally the objective of a process engineer designing a distillation scheme is to make it more efficient by reducing the heat requirement of the distillation columns within the distillation scheme. However, it is well known that in a ternary or three component separation, attempts to reduce the heat requirement leads to the need for more heat utility at a higher temperature i.e. more heat has to be supplied by the higher temperature heat source. For an above ambient temperature distillation, this could mean that heat can be saved only if more higher temperature steam is used. A higher temperature steam is more expensive. Therefore, there is definitely a need for distillation schemes to decrease heat requirement without a need for any additional higher temperature heat utility or to allow more flexibility in adjusting the temperatures at which heat is supplied to the distillation columns. This invention fulfills this long desired need for improving the efficiency of multicomponent distillation without increased demand for more expensive utilities.
Consider the separation of a ternary mixture ABC into three product streams each enriched in one of the components. A is the most volatile component and C is the least volatile component. To separate a ternary mixture ABC into almost pure components it is required that a distillation scheme use two distillation columns. Such distillation schemes are well known in the art. There are such five well known schemes: direct sequence, indirect sequence, side rectifier, side stripper and thermally coupled columns. Each of the schemes will now be described in detail:
FIG. 1 shows a direct sequence scheme. A feed mixture containing components A, B and C (stream 10) is fed to the first column having a condenser A and a reboiler BC where it is distilled to A-enriched product (stream 70) from the top. The liquid from the bottom of this column (stream 20) is primarily a binary mixture composed of components B and C. This BC liquid stream is split into two streams. A first portion (stream 22) is fed to the second column. The second portion (stream 24) is boiled and fed as stream 26 to the bottom of the first column. B-enriched product (stream 80) and C-enriched product (stream 90) are produced from the second column having a condenser B and a reboiler C. A portion of the C-enriched bottoms liquid is boiled (stream 92) and returned to the column to provide boil-up.
FIG. 2 shows an indirect sequence scheme. Distillation of the feed mixture (stream 10) in the first column having a condenser AB and a reboiler C produces C-enriched product (stream 90) from the bottom and a primarily binary vapor mixture AB (stream 30) from the top. A portion of this saturated vapor stream is fed to the second column (stream 32). Another portion (stream 34) is condensed and sent as reflux (stream 36) to the first column. A-enriched product and B-enriched product (streams 70 and 80 respectively) are produced from the second column having a condenser A and a reboiler B.
FIG. 3 shows a side rectifier scheme wherein the feed mixture ABC (stream 10) is distilled in the first column having a condenser A and a reboiler C to produce A-enriched product (stream 70) from the top and C-enriched product (stream 90) from the bottom. A portion of the C-enriched bottoms liquid is boiled (stream 92) and returned to the column to provide boil-up. The component of intermediate volatility, B, is collected (stream 80) from the top of the second column (also known as a side rectifier) having a condenser B. Notice that the second column does not have a reboiler at the bottom and instead it is fed by a vapor (stream 50) which is withdrawn from a location below the feed of the first column. This vapor stream is primarily a binary mixture consisting of components B and C. The liquid (stream 52) from, the bottom of the second column is sent to the first column at the same location as where the vapor (stream 50) was removed from the first column. This thermal coupling between the two columns reduces the number of reboilers. As compared to the schemes in FIGS. 1 and 2, the number of reboilers is reduced by one, and the total number of reboilers and condensers used are three vs. four.
FIG. 4 shows a side stripper scheme which is similar to FIG. 3 (corresponding streams and equipment use the same identification) except that the feed to the second column (now known as side stripper) is a liquid (stream 60), product B (stream 80) is collected at the bottom of the second column instead of the top and the second column has a reboiler B but no condenser. The liquid stream is withdrawn from the first column from a location which is above the feed location to the first column and is primarily a binary mixture composed of components A and B and is fed to the top of the second column. The vapor (62) from the top of the second column is returned to the first column resulting in the thermal coupling between the two columns. Notice that as compared to the scheme in FIG. 2, one less condenser is used in FIG. 4.
FIG. 5 shows a thermally coupled columns scheme which uses two thermal couplings between the first and second columns, thereby eliminating both the reboiler and condenser in the second column. The thermal coupling at the bottom of the second column is the same as the one shown in FIG. 3 and at the top is the same as the one shown in FIG. 4 (corresponding streams and equipment use the same identification). B-enriched product (stream 80) is collected from an intermediate location of the second column. Notice that due to two-thermal coupling, the total number of reboilers and condensers is reduced by two.
By now it is well known that the schemes with thermal coupling (shown in FIGS. 3-5) require less heat input than the ones without thermal coupling (FIGS. 1 and 2) (Minimum Energy Requirements of Thermally Coupled Distillation Systems, Z. T. Fidkowski and L. Krolikowski, AlChE Journal pages 643-653, volume 33, 1987). The heat requirement in the reboiler C of side rectifier in FIG. 3 is less than the total heat requirement in both reboilers BC and C of FIG. 1. Similarly, total heat input in reboilers B and C for the side stripper configuration in FIG. 4 is less than the total heat input in reboilers B and C of indirect sequence in FIG. 2. Of the five schemes, the thermally coupled configuration in FIG. 5 requires the least heat input in its reboiler.
While the heat demand decreases with thermal coupling, it comes at a cost of more expensive utilities. For example, the thermally coupled scheme in FIG. 5 requires that all the hot utility be available at the highest temperature and all the cold utility be available at the coldest temperature. In a direct sequence scheme (FIG. 1), some heat is added to reboiler BC and some to reboiler C. The temperature of reboiler BC is lower than reboiler C which implies that the heat source for reboiler BC can be at a lower temperature than the heat source for reboiler C. On the other hand, all the heat input for thermally coupled column (FIG. 5) is to reboiler C and the total heat source has to be at the higher temperature. Similarly, in direct sequence scheme (FIG. 1), some heat is removed in condenser B which is warmer than condenser A. This implies that a cold utility used for condenser B can be warmer (and hence cheaper) than the cold utility used for condenser A. On the other hand, thermally coupled columns require that all the heat be removed by the more expensive cold utility in condenser A. This effect of more expensive utilities is also observed when side rectifier (FIG. 3) and side stripper (FIG. 4) configurations are compared with schemes in FIGS. 1 and 2. This prompted Hohmann et al. to state "thermal integration by direct vapor coupling will reduce the heat load on a network while increasing the relative temperatures of the sources (hot utility) and sinks (cold utility) required" (E. C. Hohmann, M. T. Sander and H. Dunford; A New Approach to the Synthesis of Multicomponent Separation Schemes, Chem. Eng. Commun. volume 17, pages 273-284, 1982). Therefore, the major challenge is how to reduce the total heat demand without too much compromising of the temperatures of the utilities.
For schemes such as the ones shown in FIGS. 1 and 2, suggestions have been made in the literature to further reduce the demand of the more expensive utility by trading some of this demand with less expensive utility (For example, see "Two-Feed Distillation: Same Composition Feeds with Different Enthalpies", by P. C. Wankat and D. P. Kessler in Ind. Eng. Chem. Res., volume 32, pages 3061-3067, 1993). In this suggestion, when a liquid (vapor) stream containing two or more components is boiled (condensed) in a reboiler (condenser), rather than feeding just one of the saturated liquid or the saturated vapor feed to the next column, it is desired that both the streams be fed to the next columns. An example for the direct sequence configuration of FIG. 1 is shown in FIG. 6. (Corresponding streams and equipment in FIGS. 1 and 6 use the same identification.) Now a portion of the saturated vapor exiting reboiler BC is sent as a second feed (stream 28) to the second column. The total quantity of the feed to the second column in FIG. 6 is identical to the one in FIG. 1. However, by transferring a portion of the feed as vapor to the second column, heat requirement for the reboiler C is decreased but the heat input in reboiler BC is increased by the same quantity. Therefore, the total heat input is unchanged but more of it can now be provided by using a lower temperature heat source. The corresponding solution for FIG. 2 is shown in FIG. 7. (Corresponding streams and equipment in FIGS. 2 and 7 use the same identification). Now a portion of the condensed stream from condenser AB is fed as a second feed (stream 38) to the second column. By having a portion of the feed to the second column as saturated liquid, demand for the condensing duty in the cold condenser A is decreased but the condensing duty need in the warmer condenser AB is increased by the same quantity. Once again more of the cold utility at warmer temperature can be used but the total need for cold utilities remains unchanged. Clearly, there is a need for alternative solutions which can reduce the total heat demand while providing some flexibility to decrease the need for warmer hot utility and/or colder cold utility.
The same challenge exists when mixtures containing more than three components are distilled to produce product streams each enriched in one of the components. The reason being that the distillation schemes used to distill mixtures with more than three components are made up of the ternary subschemes shown in FIGS. 1 through 7. Therefore deficiencies of the ternary subschemes are also carried to the distillation of mixtures containing a greater number of components. Some known examples of four and five component distillation schemes can be found in a paper by Agrawal (Synthesis of Distillation Column Configurations for a Multicomponent Separation, Ind. Eng. Chem. Res., volume 35, pages 1059-1071, 1996).
For the distillation of binary mixtures, use of an intermediate reboiler or an intermediate condenser is well known to improve the efficiency of the distillation. (For example, see a paper by Z. T. Fidkowski and R. Agrawal, Utilization of Waste Heat Stream in Distillation, Ind. Eng. Chem. Res. volume 34, pages 1287-1293, 1995.) However, it is also known that use of an intermediate reboiler does not decrease the overall heat required for binary distillation but decreases the heat input in the bottom reboiler by the amount of heat added to the intermediate reboiler.